1. Field Of the Invention
The present invention is directed to an apparatus for removing dust, dirt, and debris from the interior of air handling ducts and similar confined areas, difficult to reach with currently available cleaning devices or systems.
2. Background Art
Because the interior of air handling ducts is not an obvious extension of the rooms of a dwelling or the work-place of a factory, business office or the like, these isolated areas are often given little attention in cleaning operations. Intensive effort is customarily made to keep living areas and working areas clean while ignoring the interior of the heating and ventilating systems. Lack of awareness of ducts as part of the living/working area is partly to blame for much of the general indifference to the cleanliness of duct systems.
Additionally, most of the cleaning systems and methods now in use are little better than those used at the turn of the century.
The broad attention being given to clean air in our society is giving strong impetus to the growth of residential and commercial duct cleanliness. Many physicians are making air duct cleanliness a feature in the treatment of their patients, specifically those with asthma. Most of the presently available duct cleaning methods are rated unsatisfactory by medical people, who must recommend them, and these medical people also have difficulty accepting the effectiveness of the best methods/systems now on the market.
Many businesses and authorities associated with duct cleaning judge the effectiveness of equipment available for this type of work by the volume of air flow in cubic feet per minute (hereinafter to be referred to as volume or air volume) that a large suction fan is capable of inducing in the duct system during the cleaning operation, whereas the proper criterion should be air velocity at the interior surface of the duct stated as feet per minute that would lift and entrain particulate matter from interior surfaces, hereinafter to be referred to as "air entrainment velocity". This air entrainment velocity is specific for different materials and it is this parameter that should be the parameter of choice used to measure and judge the effectiveness of the present invention and similar cleaning devices or systems. If the suction fan that induces air flow through the duct system being cleaned were to be sized to attain air entrainment velocity in the largest portion of duct in the system, the fan size would be inordinately large and impractical. Some examples from engineering handbooks of the entrainment velocities required for different materials are:
______________________________________ Ashes, powdered 6500-8500 fpm Cement 6500-9000 fpm Coal, powdered 4000-5500 fpm Cotton 4500-6500 fpm Flour 3500-6500 fpm Grain Dust 2000-3000 fpm ______________________________________
It can be shown, by calculation, that the volume (cfm) of air required to provide the entrainment velocity for detritus in a duct measuring 1 ft. by 1 ft., with air moving at a velocity of 5000 ft./min., would be inadequate for another section of duct in the same system measuring 2 ft. by 2 ft. since the air velocity in the larger duct would drop below the entrainment velocity. Since air ducts vary in size by design, it is therefore, logical that air entrainment velocity and not volume of air moving through the duct should be the prime criterion for rating such cleaning processes. It should be the goal of the cleaning crew to be certain that entrainment velocity is reached as close as is practical to the surface of the duct in the area being cleaned, as the cleaning operation progresses through the duct. The entrainment velocity for material varies with the size, specific gravity and the shape of the material as follows: EQU V-(6,000)(s)(d0.40)/(s+1)
in which,
V=Velocity, ft/min PA1 s=Specific Gravity of particles. PA1 d=Avg. dia. of particles, inches
Normal, laminar airflow in ducts is known to move predominately through the center of the duct in "plug flow" with air velocity, as measured across a section of the duct, being progressively slower as the interior of the duct surfaces is approached. Immediately at the interior surface of the duct the air film moves so slowly that for practical purposes it produces a boundary layer of air that can be considered, in the practical sense, to be quiescent; a condition that permits entrained dust to settle out and cling to the duct surface. Dust collection on the duct surface is less along the interior top surface of the duct and more pronounced along the bottom interior surface of the duct, the difference due to gravity. Also, the dust deposition is generally electrostatic in nature and, with time, the dust particles consolidate to a degree that air flow alone cannot satisfactorily remove this dust layer. Mechanical disturbance of the dust is requisite to both removal of the dust from the surface and to present it to swiftly moving currents of air so that said dust might be entrained and removed.
At the present state of the art, duct cleaning equipment employs high capacity suction fans set up to exhaust air directly from the household furnace plenum to a dust collecting bag chamber. These low pressure vacuum systems customarily operate at air pressures of around 0.5 psi and are most suitable for the low density material with which we are concerned in duct cleaning. Further, these currently employed systems depend upon air volume alone, as measured at the fan, to do the principal work of dust entrainment and dust transport, with some operators assisting the work of the suction fan with hand operated devices such as brushes or air jets. It has been established that such methods are helpful but inadequate. The velocity induced in the ducts by systems in current use acts only through a sweeping air current and must be assisted by, for example, mechanical devices or air jets, to overcome the quiescent boundary layer at the interior duct surfaces. The effect of this boundary layer can never be eliminated, in the practical sense, by air flow alone. It is then necessary to provide some additional means to increase the velocity of air in the duct in the local area of the cleaning apparatus. Further, since this boundary layer is a formidable barrier, it is necessary to disturb and lift the dirt and dust at the boundary layer in an energetic manner, such that the dust can be efficiently entrained in the air stream.